FIG. 1 is a sectional view of a conventional solid electrolytic capacitor (1), and FIG. 2 is an exploded perspective view of a capacitor element (2) in the solid electrolytic capacitor (1) of FIG. 1 (for example, Patent Document 1).
In this capacitor, the capacitor element (2) is housed in an aluminum case (3) opened at one end and an opening of the case (3) is sealed by a rubber sealing member (30). As shown in FIG. 2, the capacitor element (2) is composed of a wound element (21) formed by winding an anode foil (22) being aluminum foil provided with a chemical conversion coating and a cathode foil (23) being aluminum foil together into a roll with an insulating separator paper (4) sandwiched between the anode and the cathode, and a layer of solid electrolyte of such as a TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salt or the like is formed inside. As another solid electrolyte layer, conductive polymers such as polypyrrole, polythiophene, polyfuran, polyaniline, and the like can be used.
A pair of tab terminals (25), (25) are drawn from the anode foil (22) and the cathode foil (23) and leads (20), (20) are extended from the tab terminals (25), (25). The leads (20), (20) are projected outward through the sealing member (30). The periphery of the opening of the case (3) is curled to hold the sealing member (30).
When the TCNQ complex salt is used as the solid electrolyte layer, the wound element (21) is impregnated with a solution of the TCNQ complex salt dissolved by heating, and then the wound element (21) is pulled out and cooled rapidly, thereby to be solidified to form the solid electrolyte layer. And, when the foregoing solid electrolyte layer comprising the conductive polymer is formed by a chemical polymerization method, the wound element (21) is impregnated with a mixed solution obtained by adding a monomer to a solution of an oxidizing agent, and then the wound element (21) is pulled out and dried to form a solid electrolyte layer between the anode foil (22) and the cathode foil (23).
A separator (4) is paper made of natural fiber and the wound element (21) is heated to elevated temperatures of about 150 to 300° C. to be carbonized after being wound into a roll. Voids of the separator paper (4) are increased and the density of the separator paper (4) is reduced by this carbonization. When the foregoing solid electrolyte layer is formed by a chemical polymerization method, the wound element (21) is impregnated with the mixed solution obtained by adding a monomer to a solution of an oxidizing agent, and then the impregnated wound element (21) is pulled out and dried to form the solid electrolyte layer between the anode foil (22) and the cathode foil (23). Therefore, the foregoing solution or mixed solution becomes easy to penetrate into the wound element (21).
It is conceivable that the wound element (21) is formed by using a separator paper (4) having a low density originally, but in such a case, there arises a problem that the strength of paper becomes low and the separator paper is broken in winding the wound element (21). Further, when the separator paper (4) is made of natural fiber, there is a problem that the wound element (21) loses its shape if the separator paper (4) is subjected to carbonization.
As a method of solving these problems, the present applicant has proposed a method in which by using, as a separator, paper prepared by mixing synthetic fibers having a melting point higher than a carbonization temperature or not having a melting point in natural fibers, the separator paper (4) is not broken in winding the wound element (21), the wound element (21) is prevented from losing its shape during the carbonization, and the foregoing mixed solution becomes easy to penetrate into the wound element (21) (for example, Patent Document 2).
Patent Document 1: Japanese Unexamined Patent Publication No. 6-236831
Patent Document 2: Japanese Unexamined Patent Publication No. 2004-146707